DNA double strand break repair and crossover formation during meiosis are essential to ensure proper chromosome segregation. Errors in these processes can lead to chromosome mis-segregation and ultimately, aneuploidy in humans. Yet the molecular basis of the defects is not well understood. The goal of this proposal is to shed new light on the defects occurring during meiotic recombination and chromosome segregation in mammals. We identified new and important roles for Chtf18, the murine orthologue of the yeast CTF18 gene, in mammalian meiosis. CTF18 (Chromosome Transmission Fidelity factor 18) encodes an evolutionarily conserved protein that is part of the replication factor C-like complex, CTF18-RLC. CTF18 is necessary for gamete viability and accurate chromosome segregation in yeast, and it is crucial for germline development and fertility in the fruitfly. CTF18-RLC associates with chromosomes to establish cohesion between sister chromatids during DNA replication, and interacts with multi-protein complexes necessary for chromosome cohesion, called cohesins, in both yeast and in human cells in vitro. However, the exact mechanism by which CTF18-RLC establishes sister chromatid cohesion and the role CHTF18 plays in mammals have not been elucidated. Previously, we showed that Chtf18 is expressed throughout the mammalian male and female germline, suggesting a role for Chtf18 in gametogenesis. Recently, we demonstrated a role for Chtf18 in male fertility and meiosis in vivo. In Chtf18-null mice, meiotic recombination is defective and homologous chromosomes separate prematurely during prophase I. Repair of DNA double strand breaks is delayed, and these persist into diplonema. In addition, MLH1 foci (markers of DNA crossovers) are decreased in pachynema, suggesting a defect in crossover formation. Recently, we found that Chtf18-null females are subfertile, homologues separately prematurely during meiosis I, and progression of Chtf18-null oocytes through metaphase II is impaired. Thus, our data demonstrate essential roles for Chtf18 in mammalian gametogenesis and meiosis. We hypothesize that Chtf18 plays a critical role in meiosis by facilitating DNA double strand break repair and crossover formation through association with cohesin proteins. We will test our hypothesis in three experimental aims: (1): to explore the role of Chtf18 in DNA double strand break repair and crossover formation; (2): to investigate the molecular mechanisms underlying premature homologue disjunction in Chtf18-null mice; (3): to examine the consequences of Chtf18 disruption on chromosome alignment and segregation. Our proposed molecular, cell biological, and biochemical experiments will provide complementary approaches to elucidate the functions of Chtf18 in mammalian meiosis and meiotic recombination. Information gleaned from our studies will ultimately elucidate the molecular etiologies underlying chromosome mis- segregation and aneuploidy in humans.